This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-085255, filed Mar. 24, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a color cathode ray tube equipped with a shadow mask and to a color picture tube apparatus having the same.
In general, a color cathode ray tube includes a vacuum envelope which has a substantially rectangular panel and a funnel. The panel has a rectangular effective section constituting a curved surface and a skirt section at a peripheral edge of the effective section. The funnel is fixed to the skirt section. A phosphor screen is formed on the inner surface of the effective section and has three color phosphor layers and non-emitting black layers. Further, a shadow mask is arranged inside the panel and opposite to the phosphor screen. The shadow mask has a substantially rectangular mask body of a curved surface with a large number of electron beam passage apertures formed therein and a substantially rectangular mask frame supporting the peripheral edge of the mask body.
An election gun for emitting three electron beams is arranged in a neck of the funnel. The three electron beams emitted from the electron gun are deflected under magnetic field of a deflection device, mounted on the outer side of the funnel section, and horizontally/vertically scan the phosphor screen through the shadow mask, thereby displaying a color image.
In general, in order to display an image of no color drift on the phosphor screen of the color cathode ray tube, the electron beams passed through the electron beam passage apertures of the shadow mask need be landed precisely onto the three color phosphor layers of the phosphor screen. To this end, it is necessary to precisely hold a distance (q value) between the panel and the shadow mask.
In recent years, in order to improve the visibility of the color cathode ray tube and achieve a lower glaring from outer light, it is required that the curvature radius be enlarged to make the outer surface of the panel near-flat. Together with this, it is required that the curvature radius of the panel""s inner surface be enlarged even from the standpoint of visibility. Further, in the case where a proper beam landing is to be achieved on the panel""s inner surface, it is necessary to enlarge the curvature radius of the mask body where electron beam passage apertures are formed.
If, however, the curvature radius of the mask body is enlarged, then the strength of the curved surface is lowered, thus causing a deformation, etc., of the shadow mask during a manufacturing process and largely degrading the color purity of a color cathode ray tube manufactured.
In the shadow mask type color cathode ray tube, from the standpoint of an operation principle, electron beams reaching the phosphor screen past the electron beam passage apertures in the shadow mask are below ⅓ of a whole electron beam amount emitted from the electron gun. Remaining electron beams collide against the shadow mask and are converted to a heat energy to heat the shadow mask. As a result, the shadow mask is thermally expanded toward the phosphor screen side, there occurring a xe2x80x9cdomingxe2x80x9d.
If, due to the doming, a space between the phosphor screen and the shadow mask exceeds an allowable range, the electron beams are landed imprecisely on the phosphor layers and the color purity is degraded. In particular when a high brightness image pattern is locally displayed, a local doming occurs at the shadow mask and, in a shorter period of time, more imprecise beam landing occurs locally. In the case where the curvature radius of the mask body is enlarged, the above-mentioned local doming becomes prominent.
According to a cathode ray tube disclosed in U.S. Pat. No. 6,025,676, the inner surface of the panel and the mask body are formed in a semi-cylindrical curved shape wherein the radius of curvature in a direction along a long axis is set to be infinity and the radius of curvature in a direction along a short axis is set to be a certain value. By doing so, it is possible to substantially solve a mask doming problem and a mask curvature strength problem.
In the case of the above-mentioned structure, an outer light reflection between the inner surface of the panel and the phosphor screen adversely acts to a greater extent, thus lowering an image contract. This problem is alleviated by providing a selective light-pervious filter between the inner surface of the panel and the phosphor screen. If this is the case, then a high manufacturing cost is involved and a new manufacturing equipment if necessary.
Further, in the case where the contrast is to be improved without providing a filter, a panel has to be formed using glass having a transmittance of about 50%. In this case, the brightness is lower at the peripheral edge portion than at the center portion of the panel, so that the brightness uniformity is degraded.
The present invention has been contrived in consideration of the above circumstances and its object is to provide a color cathode ray tube which ensures a better external light reflection level on the inner surface of a panel and can improve color purity, and a color picture tube apparatus equipped with the color cathode ray tube.
In order to achieve the above-mentioned object, a color cathode ray tube according to the present invention comprises a vacuum envelope quipped with a substantially rectangular face panel having a substantially flat outer surface and an inner surface with a phosphor screen formed thereon, long and short axis orthogonal to a tube axis and orthogonal to each other, a pair of long sides substantially parallel to the long axis and a pair of short sides substantially parallel to the short axis; a shadow mask arranged in the vacuum envelope to face the phosphor screen; and an electron gun provided in the vacuum envelope, for emitting electron beams onto the phosphor screen, wherein the thickness of the face panel is so formed as to be greater at a peripheral edge portion than at a center portion of the f ace panel and, at the inner surface of the face panel, a curvature radius along a short axis direction on the long axis has a minimal value at a position spaced from the center of the face panel more toward a long axis end side than L/2, provided that a distance from the center of the face panel to a long axis end is L, and the face panel satisfies the following relations
TD less than 2.5TC
TV less than 2.0TC
TH less than 2.0TC
where
TC: the thickness of the face panel at a center portion
TD: the thickness of the face panel at a diagonal effective dimension end;
TV: The thickness of the face panel at a short axis effective dimension end; and
TH: the thickness of the face panel at a long axis effective dimension end.
Further, according to a color cathode ray tube of the present invention, at an area between the short axis of the face panel and the short side of the face panel, a difference between the thickness on the long axis and the thickness on the long side at a cross-section parallel to the short axis has a maximal value at a position spaced from the center of the face panel more toward the short side than L/2.
According to the color cathode ray tube of the present invention, at the inner surface of the face panel, the curvature radius in a long axis direction on the long axis has a minimal value at a position spaced from the center of the face panel more toward a long axis end side than L/2.
According to the color cathode ray tube of the present invention, at least at an area spaced from the center of the face panel L/2 toward the long axis end side, the curvature radius in a direction parallel to the short axis of the inner surface of the face panel is so set as to decrease from on the long axis toward the long side.
According to the color cathode ray tube of the present invention, the mask body of the shadow mask is also so formed as to have substantially the same shape as that of the inner surface of the face panel.
According to the thus structured color cathode ray tube and a color picture tube apparatus equipped with this color cathode ray tube, the radius of curvatures of the inner and outer surfaces of the face panel and mask body are set under proper conditions and, by doing so, it is possible to provide a better outer light reflection level on the inner surface of the face panel while ensuring a better visibility and to alleviate a lowering in color purity of the color cathode ray tube resulting from a doming of the shadow mask at a time of operating a color cathode ray tube and a lowering, etc., in color purity resulting from an imprecise beam landing caused by the deformation of the mask body at a manufacturing process.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.